Crop Rotation
Crop planning is a cornerstone of organic farming practice and it has important implications for weed management. It can be designed to positively influence weed control and to make a useful contribution to the whole farm management strategy. Typically rotation cycles extend over several years with often only an annual change of crop, but the inclusion of cover crops, intercrops and green manures increases the crop diversity in a rotation. In horticultural systems there may be sequential cropping where short-term crops follow each other in succession.
Weed population density may be markedly reduced using crop rotation but there has been little experimentation. Success depends on the use of crop sequences that create a diverse pattern of competition, allelopathic interference, soil disturbance, and production needs (such as the time of sowing and harvesting). There should be regular changes between spring and autumn-sown crops, and between annual and perennial crops, between dense leafy crops and those with an open habit, and between crops that require a long growing season and others that mature quickly. Rotation may also allow the use of a range of cultivations and direct non-chemical weeding methods that may be applicable to the different crops. The aim is to provide an unstable and inhospitable environment that prevents the proliferation of a particular weed species.
Choosing crops and their sequence
The length of the rotation, the choice and sequence of crops will depend upon individual farming circumstances that will include factors like soil type, rainfall, topography and enterprises. However, the aim is to produce an unstable environment in which no single weed species is allowed to adapt, become dominant, and therefore difficult to manage. No one rotation can be recommended, but ideally in terms of weed control rotations should include:
- alternation of autumn and spring germinating crops,
- alternation of annual and perennial crops (including grass),
- alternation of closed, dense crops such as oats which shade out weeds, and open crops such as maize which encourage weeds,
- a variety of cultivations and cutting or topping operations that directly affect the weeds.
Various suggestions and observations include:
- putting sensitive annual crops after perennial leys. Research has shown that in the third cropping year after a grass/clover ley there is twice as much weed emergence as compared to the first
- include a row crop that allows the use of one or more cultivations to kill emerged weeds and encourages the germination of others, so reducing the soil seedbank and hence potential weed numbers in future crops. Cultivations may also reduce the problem of perennial weeds by disrupting growth and smothering regeneration in the growing crop. Typical cleaning crops include turnip, sugar beet, and potato
- uncultivated leys provide a completely different habitat for weeds and may be used to reduce or eliminate particular weed species. Few studies have been made of the effectiveness of leys for controlling weeds but trials suggest that there is little advantage for weed management in leaving leys down longer than 3 years. The species composition, and the mowing and grazing regimes are important in the weed dynamics. Management of the weeds at the time of ley establishment is critical as is the method of ending the ley to avoid a flush of weeds due to the release of seed dormancy by cultivation. A greater proportion of ley in the rotation usually results in lower seed numbers in the seedbank in comparison with arable crops. It was a traditional way to deal with land infested with wild oat but does not eliminate the weed completely
- where a long grass break does not form part of the rotation weed problems are likely to be more severe. The problem will be greater where less vigorous and therefore less competitive crops are grown. Among the cereals, oats and winter rye are the most competitive followed by triticale
- canopy development and shading are important for weed suppression and choice of cultivar can influence this
- higher seeding rate and narrower row spacing increase the level of weed suppression
- competitive cereals like rye may be grown as short duration
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- Henrica 3---0-2006
- Gareth Davies 3---0-2006